Add real-time MQTT-powered LiveView dashboard

Features:
- MQTT subscriber GenServer connects to mqtt.home
- Real-time host discovery via systant/+/stats topic
- LiveView with Phoenix PubSub for instant updates
- Host cards showing live data and last seen timestamps
- Clean UI with Tailwind styling
- Proper OTP supervision tree

Dashboard ready to receive live data from systant hosts!
Visit /hosts to see real-time monitoring.

dashboard/lib/dashboard/mqtt_subscriber.ex

@@ -0,0 +1,122 @@
+defmodule Dashboard.MqttSubscriber do
+  @moduledoc """
+  MQTT subscriber that listens to systant host messages and broadcasts updates via Phoenix PubSub.
+  """
+  use GenServer
+  require Logger
+
+  alias Phoenix.PubSub
+
+  @mqtt_topic "systant/+/stats"
+  @pubsub_topic "systant:hosts"
+
+  def start_link(opts) do
+    GenServer.start_link(__MODULE__, opts, name: __MODULE__)
+  end
+
+  def get_hosts do
+    GenServer.call(__MODULE__, :get_hosts)
+  end
+
+  @impl true
+  def init(_opts) do
+    # Start MQTT connection
+    client_id = "systant_dashboard_#{System.unique_integer([:positive])}"
+    
+    connection_opts = [
+      client_id: client_id,
+      server: {Tortoise.Transport.Tcp, host: "mqtt.home", port: 1883},
+      handler: {__MODULE__, []}
+    ]
+
+    case Tortoise.Supervisor.start_child(client_id, connection_opts) do
+      {:ok, _pid} ->
+        Logger.info("MQTT subscriber connected as #{client_id}")
+        
+        # Subscribe to systant stats topic
+        Tortoise.Connection.subscribe(client_id, @mqtt_topic, qos: 0)
+        
+        {:ok, %{client_id: client_id, hosts: %{}}}
+      
+      {:error, reason} ->
+        Logger.error("Failed to start MQTT connection: #{inspect(reason)}")
+        {:stop, reason}
+    end
+  end
+
+  @impl true
+  def handle_call(:get_hosts, _from, state) do
+    {:reply, state.hosts, state}
+  end
+
+  @impl true
+  def handle_info({:tortoise, {:publish, @mqtt_topic, payload, _opts}}, state) do
+    case Jason.decode(payload) do
+      {:ok, data} ->
+        # Extract hostname from topic
+        hostname = extract_hostname_from_topic(@mqtt_topic)
+        
+        # Update host data with timestamp
+        host_data = Map.put(data, "last_seen", DateTime.utc_now())
+        updated_hosts = Map.put(state.hosts, hostname, host_data)
+        
+        # Broadcast update via PubSub
+        PubSub.broadcast(Dashboard.PubSub, @pubsub_topic, {:host_update, hostname, host_data})
+        
+        Logger.debug("Received update from #{hostname}: #{inspect(data)}")
+        
+        {:noreply, %{state | hosts: updated_hosts}}
+      
+      {:error, reason} ->
+        Logger.warning("Failed to decode JSON payload: #{inspect(reason)}")
+        {:noreply, state}
+    end
+  end
+
+  def handle_info({:tortoise, {:publish, topic, payload, _opts}}, state) do
+    # Extract hostname from the actual topic
+    case String.split(topic, "/") do
+      ["systant", hostname, "stats"] ->
+        case Jason.decode(payload) do
+          {:ok, data} ->
+            # Update host data with timestamp
+            host_data = Map.put(data, "last_seen", DateTime.utc_now())
+            updated_hosts = Map.put(state.hosts, hostname, host_data)
+            
+            # Broadcast update via PubSub
+            PubSub.broadcast(Dashboard.PubSub, @pubsub_topic, {:host_update, hostname, host_data})
+            
+            Logger.debug("Received update from #{hostname}: #{inspect(data)}")
+            
+            {:noreply, %{state | hosts: updated_hosts}}
+          
+          {:error, reason} ->
+            Logger.warning("Failed to decode JSON payload: #{inspect(reason)}")
+            {:noreply, state}
+        end
+      
+      _ ->
+        Logger.debug("Received message on unexpected topic: #{topic}")
+        {:noreply, state}
+    end
+  end
+
+  def handle_info({:tortoise, _msg}, state) do
+    # Handle other tortoise messages (connection status, etc.)
+    {:noreply, state}
+  end
+
+  # Private functions
+
+  defp extract_hostname_from_topic(topic) do
+    case String.split(topic, "/") do
+      ["systant", hostname, "stats"] -> hostname
+      _ -> "unknown"
+    end
+  end
+
+  # Tortoise handler callbacks (required when using handler: {module, args})
+  def connection(_status, _state), do: []
+  def subscription(_status, _topic_filter, _state), do: []
+  def terminate(_reason, _state), do: []
+end